Diamond surfaces

ABSTRACT

A diamond grit surface is formed on a substrate ( 1 ) having a metal surface ( 2 ), such as nickel, by applying a paste ( 4 ) of low-grade diamond grit in a binder to the surface. After driving off the binder, the diamond coated surface is placed in a reactor chamber ( 10 ) having a microwave plasma reactor ( 11 ) and connected to a hydrogen gas pump ( 12 ). The substrate ( 1 ) is heated in the hydrogen atmosphere at a reduced pressure. The metal surface ( 2 ) acts as a catalyst in the presence of the hydrogen plasma to cause regrowth of the diamond ( 6 ), giving an improved size, shape and adhesion. The method may be used to make diamond surfaces in electron emitter devices, circuit boards or abrasive devices.

FIELD OF THE INVENTION

This invention relates to methods of forming diamond surfaces and todevices having diamond surfaces formed thereon.

BACKGROUND OF THE INVENTION

Diamond is a particularly useful material for forming electron emitters,because of its low work function and crystalline nature. Various methodshave been proposed for forming diamond emitters. In the CVD (chemicalvapor deposition) process, a gaseous carbon source at high temperatureand pressure is used to deposit diamond onto a metal surface, such as ofcopper or nickel. It has also been proposed to use a solid carbon sourceat low pressure to deposit diamond onto a metal surface. A differenttechnique is described in Journal Vacuum Science Technology, B 14(3),May/June 1996 pp 2060 to 2067. In this, a diamond grit is applied to asilicon substrate in a slurry or paste, and this is then heated in areducing atmosphere to form electrical and mechanical contact betweenthe diamond and silicon substrate. A layer of nickel may then bedeposited on top of the diamond grit. EP-A-0718864 describes afield-emission device with diamond particles on a metal surface. Diamondis also extensively used in other applications, such as, as an abrasive.

It is an object of the present invention to provide an improved methodof forming a diamond surface and a device having a diamond surfacethereon.

According to one aspect of the present invention there is provided amethod of forming a diamond surface comprising the steps of providing ametal surface, depositing a layer of diamond grit onto the surface andsubjecting the surface and diamond grit to a gaseous atmosphere atelevated temperature, the metal surface and gaseous atmosphere beingselected to cause a catalytic regrowth of the diamond grit and the metalsurface being sufficiently thin that the metal surface is consumed bythe treatment in the gaseous atmosphere.

The metal surface may be provided on an electrically-insulativesubstrate, such as selected from a group comprising silicon, quartzaluminum and glass. The diamond grit is preferably applied to the metalsurface in a binder.

According to another aspect of the present invention there is provided amethod of forming a diamond surface comprising the steps of providing asurface, depositing a mixture of a diamond grit and a metal powder ontothe surface and subjecting the surface and deposited mixture to agaseous atmosphere at elevated temperature, the metal powder and gaseousatmosphere being selected to cause a catalytic regrowth of the diamondgrit.

The surface is preferably of an electrically-insulative material and themetal is preferably selected from a group comprising nicktel, silver,copper, titanium chromium, gold, lanthanum, cesium, magnesium, barium,aluminum and molybdenum. The diamond grit when applied preferably has aparticle size in the range of a few tenths of a micron to a few microns.The gaseous atmosphere is preferably of a hydrogen-containing gas, theelevated temperature is preferably in the range 500 to 1000° C. and thegaseous atmosphere preferably has a pressure in the range of a few torrto several hundred torr. The gaseous atmosphere at elevated temperatureis preferably produced by a microwave plasma reactor. The diamond gritmay be doped with an n-type or p-type material. The diamond grit may beproduced by crushing n-type or p-type CVD diamond film.

BRIEF DESCRIPTION OF THE FIGURES

A method of forming a diamond surface and a device with a diamondsurface so formed, will now be described, by way of example, withreference to the accompanying drawings in which:

FIGS. 1 to 4 are simplified sectional side elevation views of differentstages in the method of manufacture of the device, not to scale;

FIG. 5 is a sectional side elevation view of a display device in a diodeconfiguration;

FIG. 6 is a sectional side elevation view of a display device in atriode configuration; and

FIGS. 7 and 8 are sectional side elevation views of two differentarrangements of displays in which the emitter has surface conductionmodes.

DETAILED DESCRIPTION

With reference to FIG. 1. there is provided an electrically-insulativesubstrate 1. sUcli as of silicon, quartz, alumina or glass, with twometal tracks 2 formed on its upper surface by any conventionaltechnique, such as by deposition through a photo resist mask that issubsequently removed. The tracks 2 are about 1-50 μm thick and may, forexample be of nickel, or any other metal effective to produce thecatalytic action referred to below, such as silver, copper, titanium,chromium, gold, lanthanum, cesium, magnesium, aluminum or molybdenum.

In the next stage, shown in FIG. 2, a silk screen 3 is placed on theupper surface of the substrate, the screen having a pattern of porousregions 13 corresponding to the location of the tracks 2. A paste 4 isthen wiped over the upper surface of the screen 3 with a squeegee 5 sothat the paste is pushed through the porous regions 13 onto the uppersurface of the metal tracks 2. The paste 4 comprises a low-cost diamondgrit, or nanogrit, with a particle size in the range of a few tenths ofa micron to a few micron in a binder. The screen 3 is then removed andthe substrate 1 with its paste-coated tracks 2 is placed in an oven todrive off the binder and leave the diamond grit 6, as shown in FIG. 3.Typically the density of diamond git 6 on the tracks 2 is about 10,000per cm². Alternatively, the diamond grit could be deposited byultrasonic abrasion, to embed it into the upper surface of the metaltracks.

The pattern of diamond grit could be produced on the surface by anyconventional lithography technique, such as using electron-beam or X-raybeam techniques. The diamond grit could be coated by a spin-ontechnique, by printing or by electrophoresis.

In the next step, as shown in FIG. 4, the substrate 1 is placed in areactor chamber 10. The chamber 10 has a 6 kW microwave plasma reactor11 and is connected to a hydrogen gas pump 12. The reactor 11 heats thesubstrate 1 to a temperature in the range 500-1000° C. and the pump 12circulates hydrogen through the chamber 10 at a pressure in the range ofa few tenths of a torr to several hundred torr for up to about 1 hour.Other forms of hydrogen plasma could be used and the plasma couldcontain hydrocarbon gases, such as methane. Other forms of heating couldbe used, such as RF heating.

The metal of the tracks 2 acts as a catalyst in the presence of thehydrogen plasma to cause regrowth of the diamond grit 6 into highquality, faceted, sharp-edged diamond grains of increased size, in thesubmicron to micron range. It also promotes the interface states formedbetween the diamond grit and residual metal or carbides, such ascarbides formed by reaction between the metal and diamond (this isparticularly applicable where the metal is molybdenum or titanium). Theregrowth of the diamond, therefore, leads to an increase in the qualityof the diamond because of the increased size and refined shape. Theadhesion, and hence the electrical contact of the diamond with theunderlying surface is also improved. The method also has the advantagethat the regrowth of the diamond can be achieved relatively quickly.

The diamond grit 6 may be doped before applying to the tracks, with ann-type or p-type material. Altematively, the diamond may be doped afterapplying it to the tracks such as by including the dopant in thehydrogen plasma treatment. This avoids the need for any additionalsurface treatments or metal coating to enhance electron emission.Diamond grit doped in this way exhibits a desirable depletion layer atthe interface between the diamond crystal and the metal layer. Dopeddiamond grit could be produced by crushing n-type or p-type CVD diamondfilm. This has been found to enable a ready control of conductivity.

Instead of depositing the diamond nanogrit onto a preformed metalcatalyst layer, the metal catalyst could be in powder form mixed withthe diamond nanogrit and this powder mixture deposited onto a surface.The powder could be a metal oxide or other metal-containing powder.

It can be desirable, where the diamond grit is deposited onto a metallayer formed on a silicon substrate, for the metal layer to besufficiently thin that it is consumed by the regrowth process. Thisenables the diamond particle to sink into physical contact with theunderlying substrate.

The completed device may be used in the cathode of a field emissiondevice, such as a display of the kind described in GCB2297862. Thediamond surface could be used in planar emitter arrays, such as indisplay devices. The diamond surface could be applied to a cold cathodeelectrode such as in an LCD backlight.

FIGS. 5 to 8 show a device 20 to 20′″ in different arrangements oflight-emitting displays. The arrangement of FIG. 5 is a diodeconfiguration with the device 20 supported on a cathode plate 21 beneatha transparent anode plate n having a low voltage phosphor layer 23 onits lower surface. The upper, exposed surface of the device 20 faces,and is spaced opposite, the phosphor 23, being separated from it by avacuum. Electrons emitted by the device 20 are attracted to the anodeplate 22, by a voltage of between about 500 and 1000 volts, and causethe phosphor layer 23 to fluoresce. It will be appreciated that thedisplay would typically incorporate many such devices 20 so that thedesired display representation can be produced by appropriatelyaddressing individual devices in a conventional way.

The arrangement of FIG. 6 is similar to that of FIG. 5 but is a triodearrangement with a gate plate 24′ between the cathode plate 21′ and theanode plate 22′. By controlling the voltage on the gate plate 24′, theflow of electrons from the device 20′ to the anode plate 2′ can becontrolled.

The arrangement of FIG. 7 has a device 20″ operating in avoltage-biased, surface conduction mode. Two electrodes 25″ and 26″ atopposite ends of the device 20″ are connected to a low voltage dc source(approximately 14 volts), such as a battery 27″.

The arrangement of FIG. 8 also has a device 20′″ operated in avoltage-biased. surface conduction mode. In this arrangement. However,the phosphor 23′″ is not located opposite the device 20′″ but is mountedon a common insulative plate 25′″ with the device in the same plane. Theexposed surface of the fluorescent phosphor 23′″, therefore, faces inthe same direction as that of the device 20′″. Electrons emitted by thedevice 20′″ follow a curved trajectory initially moving away from theplate 28′″ and then moving back towards it under the attractive force ofthe positive voltage on the anode 22′″. This configuration has found tohave several advantages in electron emitter displays in general comparedwith conventional configurations where the phosphor is located directlyabove the electron emitter. In particular, it avoids the need forspacers to maintain accurate spacing between opposite anode and cathodeplates, it reduces the risk of contamination between the phosphor andelectron emitter, and it makes registration between the phosphor regionand the emitter easier, which is especially important in multi-colourdisplays.

The invention could also be used to form diamond surfaces on otherdevices such as on printed circuit boards where a conventionalcopper/Invar/copper layer could be replaced with a copper/diamondcomposite layer. The diamond layer has a very high thermal conductivity,making it particularly useful in applications where there is highthermal emission.

The method could be used to form abrasive devices. A diamond nanogritabrasive device formed in this way has an increased effective lifecompared with conventional diamond grit abrasive devices. An abrasivepad formed in this way could be used, for example, in multiple metallayer CMP (chemical metal polishing) planarization processes.

What is claimed is:
 1. A method of forming a diamond surface comprisingthe steps of providing a metal surface (2), depositing a layer ofdiamond grit (6) onto the surface and subjecting the surface and diamondgrit to a gaseous atmosphere at elevated temperature, the metal surfaceand gaseous atmosphere being selected to cause a catalytic regrowth ofthe diamond grit, and the metal surface being sufficiently thin that themetal surface is consumed by the treatment in the gaseous atmosphere. 2.A method according to claim 1, characterized in that the metal surface(2) is provided on an electrically-insulative substrate (1).
 3. A methodaccording to claim 2, characterized in that the metal surface (2) isprovided on a substrate (1) selected from a group comprising silicon,quartz, aluminum and glass.
 4. A method according to claim 1,characterized in that the diamond grit (6) is applied to the metalsurface as a paste (4) in a binder.
 5. A method according to claim 1,characterized in that the metal (2) is selected from a group comprisingnickel, silver, copper, titanium, chromiumn, gold, lanthanum, cesium,magnesium, barium, aluminum and molybdenum.
 6. A method according toclaim 1, characterized in that the diamond grit (6) when applied has aparticle size in the range of a few tenths of a micron to a few microns.7. A method according to claim 1, characterized in that the gaseousatmosphere is substantially of a hydrogen-containing gas.
 8. A methodaccording to claim 1, characterized in that the elevated temperature isin the range 500 to 1000° C.
 9. A method according to claim 1,characterized in that the gaseous atmosphere has a pressure in the rangeof a few torr to several hundred torr.
 10. A method according to claim1, characterized in that the gaseous atmosphere at elevated temperatureis produced by a microwave plasma reactor (11).
 11. A method accordingto claim 1, characterized in that the diamond grit (6) is doped with ann-type or p-type material.
 12. A method according to claim 1,characterized in that the diamond grit (6) is produced by crushingn-type or p-type CVD diamond film.